This invention relates to the production of cables, and more particularly is concerned with producing cables having greatly improved electrical properties in applications where high frequency (e.g. radio frequency and microwave) electrical signals are involved. In addition, this invention provides a method of producing such cables in an environmentally safe manner without using chlorofluorocarbons.
Cables of the type designed for carrying high frequency RF and microwave signals usually comprise a core having one or more inner conductors, surrounded by a dielectric, with an outer conductor or shield surrounding the dielectric. The inner conductor or conductors and the outer conductor are made of an appropriate conductive metal, e.g., copper, aluminum and various alloys, and the dielectric is usually composed of a foamed polymer such as polyethylene or a fluoropolymer.
The core of the cable is most commonly produced by extruding a mixture of the polymer and a volatile blowing agent around the inner conductor or conductors. The volatile blowing agent is injected into the extruder barrel and mixed with the polymer under the pressure of the extruder. Upon emerging from the extruder, the blowing agent forms a gas which controllably expands the polymer to create a polymer foam of a fine, uniform and closed cell structure. Chlorofluorocarbon compounds have been the most commonly used types of blowing agents for producing foam dielectrics from olefins and fluoropolymers, where the desired expansion ratio of the foam is greater than 2. U.S. Pat. Nos. 4,104,481 and 4,107,354 to Wilkenloh, et al., for example, disclose methods of forming cables with a foam polyethylene dielectric using chlorofluorocarbon (CFC) blowing agents such as CFC-12, CFC-114, CFC-113, CFC-11, and mixtures of these gases. CFC-22 (CHClF.sub.2 ; chlorodifluoromethane) is another commonly used chlorofluorocarbon blowing agent.
Although the foamed polymer dielectric materials have acceptable insulating properties (i.e. a low dielectric constant), the inherent dissipation factor, tan .delta., of the dielectric material causes undesired attenuation of the electrical signal at the high RF and microwave operating frequencies of the cables. This power loss, which is sometimes referred to as "dielectric loss", contributes to the dissipation of the electrical signal. Efforts have been made to improve the signal dissipation of cables by improvements in the dielectric loss properties of the polymer from which the foam dielectric is produced, and a number of specialized polymers have been developed for this purpose. However, it has been discovered that the blowing agents used in producing the foam structure also contribute to the undesired dielectric loss. The blowing agent gas, which remains trapped in the cells of the polymer, has its own dissipation factor, tan .delta., which contributes to the dielectric loss of the foam dielectric. In addition, the blowing agent may thermally decompose under the extrusion conditions, and the decomposition products, or compounds or radicals formed from the interaction of the decomposition products and the foam dielectric, may also contribute to the undesired signal dissipation. Any one of these mechanisms has severe adverse effects on the signal dissipation properties of the foam dielectric. This problem is especially troublesome with fluoropolymers, such as fluorinated ethylene-propylene (FEP) polymers. Since these polymers are extruded at significantly higher temperatures than polyethylene (about 600.degree. F. compared to about 300.degree. F.), the higher temperature is more likely to degrade the blowing agent into electrically deleterious decomposition products.
With the foregoing in mind, it is an important object of the present invention to provide improvements in the electrical signal dissipation properties of high frequency cables. More particularly, an object of this invention is to provide improvements in the electrical signal dissipation properties of high frequency cables of the type having a dielectric material of a foamed polymer such as a polyolefin or fluoropolymer.